FIG. 7 is a schematic diagram showing the composition of a conventional bare chip mounter. A substrate 103 is mounted on the upper surface of an XY table 104. The XY table 104 moves the substrate 103 mounted thereon in the perpendicular direction along the horizontal plane. A suction nozzle unit 102 is connected to a vacuum generator 105. The upper surface of a bare chip 101 is suctioned to the front tip of the suction nozzle unit 102 due to the suctioning force of the vacuum generator 105.
A movable part 106b of a first single-axis table holds the suction nozzle unit 102. A fixing part 106a of the first single-axis table holds the movable part 106b movably in the vertical direction. The vertical (upward/downward) movement of the suction nozzle unit 106 is guided by the first single-axis table.
A bracket 107 holds the first single-axis table 106. A second single-axis table 113 holds the bracket 107. The second single-axis table 113 is moved in the vertical direction by a motor (not illustrated). The bracket 107 is moved upwards or downwards by the movement of the second single-axis table 113 in the vertical direction.
Furthermore, the bracket 107 holds a load cell 110. The pressure-sensitive surface of the load cell 110 is connected with the upper surface of a rod 109. The lower surface of the rod 109 is connected with the upper surface of a compression spring 108. The compression spring 108 presses the suction nozzle unit 102 downwards.
A pressure determination unit 112 outputs the determined pressure of the load cell 110 to a table controller 114. The table controller 114 drives the motor (not shown) which moves the second single-axis table 113 in the vertical direction, on the basis of the output from the pressure determination unit 112. In other words, the table controller 114 controls the upward/downward movement of the second single-axis table 113 on the basis of the output of the pressure determination unit 112.
Furthermore, the bracket 107 holds the upper end of a tensile spring 111. The lower end of the tensile spring 111 is connected with the upper surface of the movable part 106b of the first single-axis table. The tensile spring 111 generates a load equal to or greater than the weight of the movable part 106b of the first single-axis table, in a direction opposite to the load direction of the compression spring 108. In other words, the tensile spring 111 lifts up the movable part 106b of the first single-axis table.
In the conventional bare chip mounter composed in this way, when the bare chip 101 suctioned to the suction nozzle unit 102 is mounted on the printed pattern of the substrate 103, the second single-axis table 113 is moved downwards. In this case, the second single-axis table 113 is moved downwards through a pushing distance A in addition to a movement distance B before contacting the bare chip 101. Furthermore, in this case, the table controller 114 compares the determined pressure of the load cell 110 with a prescribed reference applied pressure.
Here, the reference applied pressure is a value obtained by adding a load generated by the compression spring 108 and the tensile spring 111 due to the pushing distance A, to an initial determination load determined by the load cell 110 before the bare chip 101 suctioned to the suction nozzle unit 102 comes into contact with the substrate 103.
When the second single-axis table 113 is moved downwards further through the pushing distance A after the bare chip 101 comes into contact with the substrate 103, the movable part 106b of the first single-axis table is moved upwards with respect to the second single-axis table 113. Due to this upward movement of the movable part 106b, the compression spring 108 is compressed and the tensile spring 111 is returned so as to generate the reference applied pressure described above.
Furthermore, when the second single-axis table 113 is moved downwards through the pushing distance A, the bare chip 101 is pressurized by the load generated by the compression spring 108 and the tensile spring 111 due to this pushing distance A. In other words, the load generated by the compression spring 108 and the tensile spring 111 due to the pushing distance A is a mounting applied pressure.
The table controller 114 halts the operation of the second single-axis table 113 at the time that an applied pressure determined by the pressure determination unit 112 reaches the reference applied pressure.
As described above, in the conventional bare chip mounter, the load generated by the compression spring and the tensile spring due to the pushing distance becomes the mounting applied pressure (see, for example, Patent Document 1).
However, in the conventional bare chip mounter described above, a gravitational balance is achieved by the tensile spring 111. Therefore, if thermal expansion occurs on members connected to the tensile spring 111 (the movable part 106b of the first single-axis table, the suction bit section 102, the bare chip 101, and the like), then the tensile spring 111 expands or contracts due to this thermal expansion and the gravitational balance is disturbed. This disturbance of the gravitational balance inhibits precise determination of the mounting applied pressure.    Patent Document 1: Japanese Patent Application Publication No. 8-330790